[gs-devel] TIFF LZW Compression

Ken Sharp ken.sharp at artifex.com
Wed Dec 15 08:06:27 UTC 2010

At 23:34 14/12/2010 +0000, Eric Burcham wrote:

>I have attached two small images.  One is made with the tifflzw output, 
>the other with tiffgray.  No other settings were changed.  You can see 
>there is some cross-hatch looking pattern all over the lzw one.  I'm 
>getting this on all outputs other than the non-compressed ones.  The flaw 
>looks fundamentally the same in all cases.

It looks to me like the tifflzw has a screen (or possibly an ordered 
dither, but it looks more like a halftone) applied to the output, looks 
like a 5x5 cell.

A halftone would make sense, since the tifflzw device is a 1 bit per pixel 
output. What doesn't make sense is the fact that it contains grayscale 
pixels. I'm completely unable to reproduce this, even using your command 
line, for me the tifflzw output is always 1 bit per pixel.

Obviously these JPEG files are not the actual output from GS, and I'm not 
experimenting with your original input file, so this is not a good comparison.

I'd suggest you reproduce the problem with the command line Ghostscript, 
and raise this as a bug report, attaching the original input to the bug.


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